5.3: Calculate kinetic energy, gravitational potential energy, and elastic potential energy. Do Now: 1. Hand in your Forms of Energy Wheel

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1 Do Now: 1. Hand in your Forms of Energy Wheel 2. Identify the following forms of energy: a. A hiker at the top of a mountain b. A dog chasing a cat c. A rubber band being stretched

2 Agenda: How can we calculate kinetic energy? gravitational potential energy? and elastic potential energy? Do Now Agenda The Formulas and Practice Wrap Up

3 Kinetic Energy: Energy that comes from motion KE = 1 2 mv2 KE stands for Kinetic Energy and has units of Joules (J) m stands for mass and has units of kilograms (kg) v stands for velocity and has units of meters per second (m/s)

4 What is the kinetic energy of a 2000 kg car moving at 45 m/s (~100 mph)? Givens: KE=? m=200 kg v=45 m/s KE = 1 2 mv2 KE = 1 2 KE = kg kg 452 KE = KE = J m s 2 m2 s 2 kg m2 s KE = 1 2 mv2

5 A 7.00 kg bowling ball moves at 2.00 m/s. How fast must a 2.45 g ping pong tennis ball move in order to have the same kinetic energy as the bowling ball? Givens: v pingpong =? m bowling =7.00 kg v bowling =2.00 m/s m pingpong =2.45 g =2.45 g * kg = kg KE pingpong = KE bowling 1 2 m 2 pingpongv pingpong 2 m pingpong v pingpong 2 v pingpong 2 v pingpong = m 2 bowlingv bowling m pingpong = 7.00kg 2.00 m s kg = 1 2 m 2 bowlingv bowling 2 = m bowling v bowling 2 2 v pingpong = m2 s 2 KE = 1 2 mv2 v pingpong = m2 s 2 = m s

6 PE graviational stands for gravitational potential energy and has units of Joules (J) m stands for mass and has units of kilograms (kg) g stands for acceleration due to gravity, (9.8), and has units of meters per second squared (m/s 2 ) h stands for height and has units of meters (m) Gravitational Potential Energy: Energy that comes from being at some height (usually above the Earth) PE graviational = mgh

7 What is the Gravitational Potential Energy of a 80 kg (~176 lbs) skydiver 3810 m (12500 ft) above the ground? Givens: PE gravitational =? m=80 kg h=3810 m PE graviational = mgh PE gravitational = (80kg)(9.8 m s 2)(3810m) kg m2 PE graviational = s PE graviational = J PE graviational = mgh

8 A 40.0 kg child is on a swing with ropes 2.00m long. Find the gravitational potential energy when a. she s at the very top of the swing PE graviational = mgh b. she s at the very bottom of the swing PE gravitational = (40.0kg)(9.8 m s 2)(2m) Givens: PE gravitational =? m=40.0 kg h=2 m PE graviational = 196 PE graviational = 196 J kg m2 s PE graviational = mgh

9 A 40.0 kg child is on a swing with ropes 2.00m long. Find the gravitational potential energy when b. she s at the very bottom of the swing PE graviational = mgh PE gravitational = (40.0kg)(9.8 m s 2)(0m) Givens: PE gravitational =? m=40.0 kg h=0 m kg m2 PE graviational = 0 s PE graviational = 0J

10 Elastic (Spring) Potential Energy: Energy that comes from compressing or stretching (usually a spring) PE elastic = 1 2 kx2 Spring Constant: Units: [N/m] F spring = kx

11 PE elastic stands for elastic potential energy and has units of Joules (J) k stands for the spring constant and has units of Newtons per meter (N/m) x stands for how much the spring is stretch or compressed, and has units of meters (m) Elastic (Spring) Potential Energy: Energy that comes from compressing or stretching (usually a spring) PE elastic = 1 2 kx2

12 What is the elastic potential energy when a spring, with a spring constant of 71 N/m, is compressed 0.10 m? Givens: PE elastic =? k=71 N/m x=0.10 m PE elastic = 1 2 kx2 PE elastic = N m 0.10 m 2 PE elastic = N m2 m PE graviational = N m PE graviational = J PE elastic = 1 2 kx2

13 The staples inside a stapler are kept in place by a spring with a relaxed length of m. If the spring constant is 51.0 N/m, how much elastic potential energy is stored in the spring when its length is m? Givens: PE elastic =? k=51.0 N/m x final =0.150 m x initial =0.115 m x=x final -x initial =0.150m-0.115m =0.035 m PE elastic = 1 2 kx2 PE elastic = N m m 2 PE elastic = N m2 m PE graviational = N m PE elastic = 1 2 kx2 PE graviational = J

14 Wrap Up: EXIT TICKET: Fill in the following table: Type of Energy When do you have it? Formula Gravitational Potential Energy Stretch or compress something (usually a spring) HOMEWORK: Practice B (Pg. 166): #1-4 Practice D (Pg. 172): #1 Section Review (Pg. 172): # mv2

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